(1) Field of the Invention
The present invention relates to systems and methods of demodulation and in particular, to a demodulation system and method used in a phase generated carrier system employing quadrature carriers.
(2) Description of the Prior Art
In communication systems, modulation is commonly used to transmit information from an information source, such as a sensor system where information is detected, to an information destination, such as a receiver system where detected signals are received and processed. According to conventional modulation techniques, a signal of interest detected by a sensor modulates a carrier signal by modifying one or more characteristics of the carrier signal, such as amplitude, frequency or phase, to form a modulated carrier signal. The modulated carrier signal is then more easily transmitted over the appropriate communication channels to the destination or receiver system where the modulated carrier signal is demodulated to recover the signal of interest and determine the information.
One type of sensor system that employs modulation techniques includes fiber optic sensors, for example, as used in fiber optic interferometers designed to sense numerous signals. These signals of interest modulate the output phase of the interferometer. In a system having an array of sensors, the signals are often multiplexed, for example, using time division multiplexing (TDM), wavelength division multiplexing (WDM), and/or frequency division multiplexing (FDM).
Such interferometers can be part of a sensor system using phase generated carriers. The sensor time varying phase signal modulates the phase generated carriers to form a modulated carrier. Both the phase generated carriers and the sensor modulation can be mathematically represented as a Bessel series of harmonically related terms. Therefore, the Bessel series of the signal of interest modulates the Bessel series of the phase generated carrier. The number of terms in the Bessel series of the resulting modulated carrier will be dependent upon the level of the measured or detected signal of interest. The harmonically related terms in the Bessel series of the modulated carrier represent both the measured or detected signal of interest and the carrier signal.
The homodyne receiver concept is typically used to demodulate the sensor information or signal of interest from an adjacent pair of carriers. According to prior art demodulation techniques, a quadrature pair of modulated carriers must be multiplied by a local oscillator of the proper frequency, phase and amplitude. Matching the phase of the local oscillator with the phase of the modulated carrier is often tedious and inexact. If either the phase or amplitude are mismatched, the harmonic distortion of the demodulator will be increased.
Typical fiber optic sensor systems using phase generated carriers to transmit a detected or measured signal (i.e., signal of interest) to a receiver system have used a pair of quadrature carriers with frequencies of either xcfx890(t) and xcfx89c(t), or xcfx89c(t) and 2xcfx89c(t) , where xcfx89c is the carrier frequency. In multiplexed sensor systems, the sensor sampling frequency fs must be selected to ensure that frequencies greater than fs/2 are not aliased into the band of interest below fs/2. In order to satisfy this criteria, sampling must occur at twice the highest bandwidth of interest. For example, a minimum sampling frequency of 4xcfx89c(t) was employed to reproduce carriers of xcfx89c(t) and 2xcfx89c(t). Thus, previous sampling frequencies in quadrature carrier systems were greater than or equal to four times the frequency of the lowest frequency quadrature carrier. Such a high sampling frequency often places great demands on the sampling circuitry.
U.S. Pat. No. 5,883,548, incorporated herein by reference, discloses a demodulation system and method in which the sampling frequency is lowered to 3xcfx89c(t). Although this system increased the bandwidth relative to the sensor""s sampling frequency, there is still a need to further increase the bandwidth.
Another system and method for recovering a signal of interest from a phase modulated signal using quadrature sampling is disclosed in U.S. Pat. No. 5,923,030, also incorporated herein by reference.
Accordingly, one object of the present invention is to provide a system and method for recovering a measured or detected signal of interest from the quadrature components of a single modulated carrier.
Another object of the present invention is to maximize the sensors"" demodulated bandwidth relative to the sensor system sampling frequency.
The present invention features a system and method of recovering at least one signal of interest that is detected by at least one sensor and that modulates a carrier signal to form a modulated carrier signal having the form
f(t)=A+B sin[C1 cos(xcfx89c(t)+xcfx86c)+(C2 cos(2xcfx89c(t)+xcfx86c)+(H cos(xcfx89h(t)+xcfx86(t))))]
where A and B are detection factors, C1 is the amplitude of the carrier signal at xcfx89c, C2 is the amplitude of the carrier signal at 2xcfx89c, xcfx89c(t) is the carrier signal radian frequency, xcfx86c is the phase of the carrier signal relative to the sensor, H is amplitude of the signal of interest, xcfx89h(t) is the radian frequency of signal of interest, and xcfx86(t) is environmentally induced phase noise.
The method comprises the steps of: receiving the sampled, modulated carrier signal; orthogonally demodulating the sampled, modulated carrier signal by multiplying the modulated carrier signal by cos (2xcfx89c(t0)) to extract an even harmonic component of the signal of interest and multiplying the modulated carrier by sin(2xcfx89c(t0)) to extract an odd harmonic component of the signal of interest; and further demodulating the even harmonic component and the odd harmonic component to recover the signal of interest. According to the preferred method, the modulated carrier is undersampled at two times the carrier frequency (fs=2xcfx89c).
The step of further demodulating the even harmonic component and odd harmonic component of the signal of interest preferably includes: taking the square root of the sum of the squares of the even harmonic component and the odd harmonic component; normalizing the even harmonic component and the odd harmonic component to form a normalized even harmonic component and a normalized odd harmonic component; and decoding the normalized even harmonic component and the normalized odd harmonic component, to recover the signal of interest. The step of decoding preferably includes: differentiating and cross-multiplying the normalized even harmonic component and said normalized odd harmonic component to form differentiated and cross-multiplied even and odd harmonic components; and differencing the differentiated and cross-multiplied even and odd harmonic components, to recover the signal of interest.
In a system having an array of sensors, the preferred method also includes multiplexing the modulated carrier signals using, for example, time division multiplexing (TDM), wavelength division multiplexing (WDM), frequency division multiplexing (FDM), or a combination of these. According to one embodiment, the sensor includes a fiber optic sensor in an array of fiber optic sensors, such as interferometers. In this embodiment the detection factor A represents the DC level of the light and the detection factor B represents the mixing efficiency of the sensor and the intensity of light.
The system for recovering the signal of interest from the modulated carrier according to the present invention includes sampling and demodulation circuitry for performing the above method.